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                        <h1 id="let%E5%92%8Cconst%E5%91%BD%E4%BB%A4">let&#x548C;const&#x547D;&#x4EE4;</h1>
<h2 id="let%E5%91%BD%E4%BB%A4">let&#x547D;&#x4EE4;</h2>
<p>ES6&#x65B0;&#x589E;&#x4E86;let&#x547D;&#x4EE4;&#xFF0C;&#x7528;&#x6765;&#x58F0;&#x660E;&#x53D8;&#x91CF;&#x3002;&#x5B83;&#x7684;&#x7528;&#x6CD5;&#x7C7B;&#x4F3C;&#x4E8E;var&#xFF0C;&#x4F46;&#x662F;&#x6240;&#x58F0;&#x660E;&#x7684;&#x53D8;&#x91CF;&#xFF0C;&#x53EA;&#x5728;let&#x547D;&#x4EE4;&#x6240;&#x5728;&#x7684;&#x4EE3;&#x7801;&#x5757;&#x5185;&#x6709;&#x6548;&#x3002;</p>
<pre><code class="lang-javascript">
{
    <span class="hljs-keyword">let</span> a = <span class="hljs-number">10</span>;
    <span class="hljs-keyword">var</span> b = <span class="hljs-number">1</span>;
}

a <span class="hljs-comment">// ReferenceError: a is not defined.</span>
b <span class="hljs-comment">//1</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5728;&#x4EE3;&#x7801;&#x5757;&#x4E4B;&#x4E2D;&#xFF0C;&#x5206;&#x522B;&#x7528;let&#x548C;var&#x58F0;&#x660E;&#x4E86;&#x4E24;&#x4E2A;&#x53D8;&#x91CF;&#x3002;&#x7136;&#x540E;&#x5728;&#x4EE3;&#x7801;&#x5757;&#x4E4B;&#x5916;&#x8C03;&#x7528;&#x8FD9;&#x4E24;&#x4E2A;&#x53D8;&#x91CF;&#xFF0C;&#x7ED3;&#x679C;let&#x58F0;&#x660E;&#x7684;&#x53D8;&#x91CF;&#x62A5;&#x9519;&#xFF0C;var&#x58F0;&#x660E;&#x7684;&#x53D8;&#x91CF;&#x8FD4;&#x56DE;&#x4E86;&#x6B63;&#x786E;&#x7684;&#x503C;&#x3002;&#x8FD9;&#x8868;&#x660E;&#xFF0C;let&#x58F0;&#x660E;&#x7684;&#x53D8;&#x91CF;&#x53EA;&#x5728;&#x5B83;&#x6240;&#x5728;&#x7684;&#x4EE3;&#x7801;&#x5757;&#x6709;&#x6548;&#x3002;</p>
<p>for&#x5FAA;&#x73AF;&#x7684;&#x8BA1;&#x6570;&#x5668;&#xFF0C;&#x5C31;&#x5F88;&#x5408;&#x9002;&#x4F7F;&#x7528;let&#x547D;&#x4EE4;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-keyword">for</span>(<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i &lt; arr.length; i++){}

<span class="hljs-built_in">console</span>.log(i)
<span class="hljs-comment">//ReferenceError: i is not defined</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x7684;&#x8BA1;&#x6570;&#x5668;i&#xFF0C;&#x53EA;&#x5728;for&#x5FAA;&#x73AF;&#x4F53;&#x5185;&#x6709;&#x6548;&#x3002;</p>
<p>&#x4E0B;&#x9762;&#x7684;&#x4EE3;&#x7801;&#x5982;&#x679C;&#x4F7F;&#x7528;var&#xFF0C;&#x6700;&#x540E;&#x8F93;&#x51FA;&#x7684;&#x662F;9&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-keyword">var</span> a = [];
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10</span>; i++) {
  <span class="hljs-keyword">var</span> c = i;
  a[i] = <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>{
    <span class="hljs-built_in">console</span>.log(c);
  };
}
a[<span class="hljs-number">6</span>](); <span class="hljs-comment">// 9</span>
</code></pre>
<p>&#x5982;&#x679C;&#x4F7F;&#x7528;let&#xFF0C;&#x58F0;&#x660E;&#x7684;&#x53D8;&#x91CF;&#x4EC5;&#x5728;&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#x5185;&#x6709;&#x6548;&#xFF0C;&#x6700;&#x540E;&#x8F93;&#x51FA;&#x7684;&#x662F;6&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-keyword">var</span> a = [];
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">var</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10</span>; i++) {
  <span class="hljs-keyword">let</span> c = i;
  a[i] = <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>{
    <span class="hljs-built_in">console</span>.log(c);
  };
}
a[<span class="hljs-number">6</span>](); <span class="hljs-comment">// 6</span>
</code></pre>
<p>let&#x4E0D;&#x50CF;var&#x90A3;&#x6837;&#xFF0C;&#x4F1A;&#x53D1;&#x751F;&#x201C;&#x53D8;&#x91CF;&#x63D0;&#x5347;&#x201D;&#x73B0;&#x8C61;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">do_something</span>(<span class="hljs-params"></span>) </span>{
  <span class="hljs-built_in">console</span>.log(foo); <span class="hljs-comment">// ReferenceError</span>
  <span class="hljs-keyword">let</span> foo = <span class="hljs-number">2</span>;
}
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5728;&#x58F0;&#x660E;foo&#x4E4B;&#x524D;&#xFF0C;&#x5C31;&#x4F7F;&#x7528;&#x8FD9;&#x4E2A;&#x53D8;&#x91CF;&#xFF0C;&#x7ED3;&#x679C;&#x4F1A;&#x629B;&#x51FA;&#x4E00;&#x4E2A;&#x9519;&#x8BEF;&#x3002;</p>
<p>&#x6CE8;&#x610F;&#xFF0C;let&#x4E0D;&#x5141;&#x8BB8;&#x5728;&#x76F8;&#x540C;&#x4F5C;&#x7528;&#x57DF;&#x5185;&#xFF0C;&#x91CD;&#x590D;&#x58F0;&#x660E;&#x540C;&#x4E00;&#x4E2A;&#x53D8;&#x91CF;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-comment">// &#x62A5;&#x9519;</span>
{
    <span class="hljs-keyword">let</span> a = <span class="hljs-number">10</span>;
    <span class="hljs-keyword">var</span> a = <span class="hljs-number">1</span>;
}

<span class="hljs-comment">// &#x62A5;&#x9519;</span>
{
    <span class="hljs-keyword">let</span> a = <span class="hljs-number">10</span>;
    <span class="hljs-keyword">let</span> a = <span class="hljs-number">1</span>;
}
</code></pre>
<h2 id="%E5%9D%97%E7%BA%A7%E4%BD%9C%E7%94%A8%E5%9F%9F">&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;</h2>
<p>let&#x5B9E;&#x9645;&#x4E0A;&#x4E3A;JavaScript&#x65B0;&#x589E;&#x4E86;&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">f1</span>(<span class="hljs-params"></span>) </span>{
  <span class="hljs-keyword">let</span> n = <span class="hljs-number">5</span>;
  <span class="hljs-keyword">if</span> (<span class="hljs-literal">true</span>) {
      <span class="hljs-keyword">let</span> n = <span class="hljs-number">10</span>;
  }
  <span class="hljs-built_in">console</span>.log(n); <span class="hljs-comment">// 5</span>
}
</code></pre>
<p>&#x4E0A;&#x9762;&#x7684;&#x51FD;&#x6570;&#x6709;&#x4E24;&#x4E2A;&#x4EE3;&#x7801;&#x5757;&#xFF0C;&#x90FD;&#x58F0;&#x660E;&#x4E86;&#x53D8;&#x91CF;n&#xFF0C;&#x8FD0;&#x884C;&#x540E;&#x8F93;&#x51FA;5&#x3002;&#x8FD9;&#x8868;&#x793A;&#x5916;&#x5C42;&#x4EE3;&#x7801;&#x5757;&#x4E0D;&#x53D7;&#x5185;&#x5C42;&#x4EE3;&#x7801;&#x5757;&#x7684;&#x5F71;&#x54CD;&#x3002;&#x5982;&#x679C;&#x4F7F;&#x7528;var&#x5B9A;&#x4E49;&#x53D8;&#x91CF;n&#xFF0C;&#x6700;&#x540E;&#x8F93;&#x51FA;&#x7684;&#x503C;&#x5C31;&#x662F;10&#x3002;</p>
<p>&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#x7684;&#x51FA;&#x73B0;&#xFF0C;&#x5B9E;&#x9645;&#x4E0A;&#x4F7F;&#x5F97;&#x83B7;&#x5F97;&#x5E7F;&#x6CDB;&#x5E94;&#x7528;&#x7684;&#x7ACB;&#x5373;&#x6267;&#x884C;&#x533F;&#x540D;&#x51FD;&#x6570;&#xFF08;IIFE&#xFF09;&#x4E0D;&#x518D;&#x5FC5;&#x8981;&#x4E86;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-comment">// IIFE&#x5199;&#x6CD5;</span>
(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>{
    <span class="hljs-keyword">var</span> tmp = ...;
    ...
}());

<span class="hljs-comment">// &#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#x5199;&#x6CD5;</span>
{
    <span class="hljs-keyword">let</span> tmp = ...;
    ...
}
</code></pre>
<p>&#x53E6;&#x5916;&#xFF0C;ES6&#x4E5F;&#x89C4;&#x5B9A;&#xFF0C;&#x51FD;&#x6570;&#x672C;&#x8EAB;&#x7684;&#x4F5C;&#x7528;&#x57DF;&#xFF0C;&#x5728;&#x5176;&#x6240;&#x5728;&#x7684;&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#x4E4B;&#x5185;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">f</span>(<span class="hljs-params"></span>) </span>{ <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&apos;I am outside!&apos;</span>); }
(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>{
  <span class="hljs-keyword">if</span>(<span class="hljs-literal">false</span>) {
    <span class="hljs-comment">// &#x91CD;&#x590D;&#x58F0;&#x660E;&#x4E00;&#x6B21;&#x51FD;&#x6570;f</span>
    <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">f</span>(<span class="hljs-params"></span>) </span>{ <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&apos;I am inside!&apos;</span>); }
  }

  f();
}());
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x5728;ES5&#x4E2D;&#x8FD0;&#x884C;&#xFF0C;&#x4F1A;&#x5F97;&#x5230;&#x201C;I am inside!&#x201D;&#xFF0C;&#x4F46;&#x662F;&#x5728;ES6&#x4E2D;&#x8FD0;&#x884C;&#xFF0C;&#x4F1A;&#x5F97;&#x5230;&#x201C;I am outside!&#x201D;&#x3002;&#x8FD9;&#x662F;&#x56E0;&#x4E3A;ES5&#x5B58;&#x5728;&#x51FD;&#x6570;&#x63D0;&#x5347;&#xFF0C;&#x4E0D;&#x7BA1;&#x4F1A;&#x4E0D;&#x4F1A;&#x8FDB;&#x5165;if&#x4EE3;&#x7801;&#x5757;&#xFF0C;&#x51FD;&#x6570;&#x58F0;&#x660E;&#x90FD;&#x4F1A;&#x63D0;&#x5347;&#x5230;&#x5F53;&#x524D;&#x4F5C;&#x7528;&#x57DF;&#x7684;&#x9876;&#x90E8;&#xFF0C;&#x5F97;&#x5230;&#x6267;&#x884C;&#xFF1B;&#x800C;ES6&#x652F;&#x6301;&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#xFF0C;&#x4E0D;&#x7BA1;&#x4F1A;&#x4E0D;&#x4F1A;&#x8FDB;&#x5165;if&#x4EE3;&#x7801;&#x5757;&#xFF0C;&#x5176;&#x5185;&#x90E8;&#x58F0;&#x660E;&#x7684;&#x51FD;&#x6570;&#x7686;&#x4E0D;&#x4F1A;&#x5F71;&#x54CD;&#x5230;&#x4F5C;&#x7528;&#x57DF;&#x7684;&#x5916;&#x90E8;&#x3002;</p>
<p>&#x9700;&#x8981;&#x6CE8;&#x610F;&#x7684;&#x662F;&#xFF0C;&#x5982;&#x679C;&#x5728;&#x4E25;&#x683C;&#x6A21;&#x5F0F;&#x4E0B;&#xFF0C;&#x51FD;&#x6570;&#x53EA;&#x80FD;&#x5728;&#x9876;&#x5C42;&#x4F5C;&#x7528;&#x57DF;&#x548C;&#x51FD;&#x6570;&#x5185;&#x58F0;&#x660E;&#xFF0C;&#x5176;&#x4ED6;&#x60C5;&#x51B5;&#xFF08;&#x6BD4;&#x5982;if&#x4EE3;&#x7801;&#x5757;&#x3001;&#x5FAA;&#x73AF;&#x4EE3;&#x7801;&#x5757;&#xFF09;&#x7684;&#x58F0;&#x660E;&#x90FD;&#x4F1A;&#x62A5;&#x9519;&#x3002;</p>
<h2 id="const%E5%91%BD%E4%BB%A4">const&#x547D;&#x4EE4;</h2>
<p>const&#x4E5F;&#x7528;&#x6765;&#x58F0;&#x660E;&#x53D8;&#x91CF;&#xFF0C;&#x4F46;&#x662F;&#x58F0;&#x660E;&#x7684;&#x662F;&#x5E38;&#x91CF;&#x3002;&#x4E00;&#x65E6;&#x58F0;&#x660E;&#xFF0C;&#x5E38;&#x91CF;&#x7684;&#x503C;&#x5C31;&#x4E0D;&#x80FD;&#x6539;&#x53D8;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-keyword">const</span> PI = <span class="hljs-number">3.1415</span>;
PI <span class="hljs-comment">// 3.1415</span>

PI = <span class="hljs-number">3</span>;
PI <span class="hljs-comment">// 3.1415</span>

<span class="hljs-keyword">const</span> PI = <span class="hljs-number">3.1</span>;
PI <span class="hljs-comment">// 3.1415</span>
</code></pre>
<p>&#x4E0A;&#x9762;&#x4EE3;&#x7801;&#x8868;&#x660E;&#x6539;&#x53D8;&#x5E38;&#x91CF;&#x7684;&#x503C;&#x662F;&#x4E0D;&#x8D77;&#x4F5C;&#x7528;&#x7684;&#x3002;&#x9700;&#x8981;&#x6CE8;&#x610F;&#x7684;&#x662F;&#xFF0C;&#x5BF9;&#x5E38;&#x91CF;&#x91CD;&#x65B0;&#x8D4B;&#x503C;&#x4E0D;&#x4F1A;&#x62A5;&#x9519;&#xFF0C;&#x53EA;&#x4F1A;&#x9ED8;&#x9ED8;&#x5730;&#x5931;&#x8D25;&#x3002;</p>
<p>const&#x7684;&#x4F5C;&#x7528;&#x57DF;&#x4E0E;let&#x547D;&#x4EE4;&#x76F8;&#x540C;&#xFF1A;&#x53EA;&#x5728;&#x58F0;&#x660E;&#x6240;&#x5728;&#x7684;&#x5757;&#x7EA7;&#x4F5C;&#x7528;&#x57DF;&#x5185;&#x6709;&#x6548;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-keyword">if</span> (condition) {
    <span class="hljs-keyword">const</span> MAX = <span class="hljs-number">5</span>;
}

<span class="hljs-comment">// &#x5E38;&#x91CF;MAX&#x5728;&#x6B64;&#x5904;&#x4E0D;&#x53EF;&#x5F97;</span>
</code></pre>
<p>const&#x58F0;&#x660E;&#x7684;&#x5E38;&#x91CF;&#xFF0C;&#x4E5F;&#x4E0E;let&#x4E00;&#x6837;&#x4E0D;&#x53EF;&#x91CD;&#x590D;&#x58F0;&#x660E;&#x3002;</p>
<pre><code class="lang-javascript">
<span class="hljs-keyword">var</span> message = <span class="hljs-string">&quot;Hello!&quot;</span>;
<span class="hljs-keyword">let</span> age = <span class="hljs-number">25</span>;

<span class="hljs-comment">// &#x4EE5;&#x4E0B;&#x4E24;&#x884C;&#x90FD;&#x4F1A;&#x62A5;&#x9519;</span>
<span class="hljs-keyword">const</span> message = <span class="hljs-string">&quot;Goodbye!&quot;</span>;
<span class="hljs-keyword">const</span> age = <span class="hljs-number">30</span>;
</code></pre>

                    
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